A common sign of meth abuse is extreme tooth decay, a condition that has become known in the media as meth mouth. Users with meth mouth have blackened, stained, or rotting teeth, which often can't be saved, even among young or short-term users. The exact causes of meth mouth are not fully understood. Various reports have attributed the decay to the corrosive effects of the chemicals found in the drug, such as anhydrous ammonia (found in fertilizers), red phosphorus (found on matchboxes) and lithium (found in batteries), which when smoked or snorted might erode the tooth's protective enamel coating; however, it's more likely that this degree of tooth decay is brought on by a combination of side effects from a meth high.